The present invention relates to a circuit configuration for the generation of a reference voltage, with a reference voltage source and a storage capacitor to which a voltage provided by a reference voltage source can be applied via a controllable switch, and whose charging voltage is the reference voltage to be generated, whereby the controllable switch is a MOS field-effect transistor with back gate which, by means of a refresh signal supplied by a control circuit, can be put periodically into either a conducting or a non-conducting state.
A circuit configuration with which the supply voltage of digital systems can be monitored is known from the application report SLVA 091 of Texas Instruments. This circuit configuration comprises a circuit section which generates the reference voltage required for the monitoring process. In an attempt to make the monitoring circuit as current saving as possible, the sample-and-hold principle is used by the circuit section for the generation of the reference voltage. This means that the reference voltage source is not continually operative, but is only switched into action periodically, each time for a short span of time. To ensure that the required reference voltage is nevertheless available on a continuous basis, it is stored in a capacitor which is connected by means of a switch during the time periods whenever the reference voltage source is active. The charging current of the capacitor is used as the required reference voltage. The switch between the reference voltage source and the capacitor is made by a MOS field-effect transistor which has a certain leakage current, leading to a discharge of the capacitor and, as a consequence, to a drop of the reference voltage stored. This leakage current therefore determines the time intervals after which the reference voltage source must be made active once again. In the circuit configuration known, no specific measures have been taken to reduce the leakage current of the MOS field-effect transistor used as switch between the reference voltage source and the capacitor.
The invention rests on the requirement of providing a circuit configuration of the type previously indicated, which supplies the reference voltage on a continuous basis and with high precision, and whose current consumption can be kept very low.
According to the invention, this requirement is satisfied in that the back gate of the MOS field-effect transistor is connected to an auxiliary storage capacitor to which the voltage supplied by the reference voltage source can be applied via a further switch, including a MOS field-effect transistor with a back gate, also controlled by the refresh signal, whereby a fixed voltage, which is greater than the voltage supplied by the reference voltage source, is applied to the back gate of the further MOS field-effect transistor.
In the circuit configuration according to the invention, the leakage current of the switch between the reference voltage source and the storage capacitor is reduced to a very low level in that the back gate of the MOS field-effect transistor used in this switch is kept at practically the same voltage level as the one supplied by the reference voltage source and which is also present at the storage capacitor. Because of the lack of any noticeable voltage difference between the back gate and the terminal of the MOS field-effect transistor connected to the storage capacitor, leakage of current through the back gate is now prevented. Since the charging voltage at the storage capacitor is thereby maintained for a long time, the time intervals, after which the reference voltage source has to be made active again, can be very long, thus resulting in a correspondingly reduced current consumption of the circuit configuration.